Tamoxifen Levels and Effects while Breastfeeding

Summary of Use during Lactation

Since tamoxifen can suppress postpartum lactation and its excretion into breastmilk is not known, it should be avoided in nursing mothers.

Drug Levels

Maternal Levels. Relevant published information was not found as of the revision date.

Infant Levels. Relevant published information was not found as of the revision date.

Effects in Breastfed Infants

Relevant published information was not found as of the revision date.

Effects on Lactation and Breastmilk

Tamoxifen was more effective than placebo in suppressing lactation and preventing engorgement and pain in two trials in postpartum mothers. Neither study stated what, if any, physical methods (e.g., breast binding) were used concurrently.[1][2] In one study of 80 women, tamoxifen 10 mg four times daily for 5 days was more effective than placebo in suppressing a rise in serum prolactin after use of a mechanical breast pump after 5 days of treatment, but not on day 3. All of the women in the study had breastfed a previous child.[2] The other study of 150 women used 2 regimens: tamoxifen 30 mg twice daily for 2 days followed by 10 mg twice daily for 2 days; and 10 mg twice daily for 14 days. More women in the tamoxifen groups had not previously breastfed an infant.[1]

In a case report, a woman with a history of breastfeeding 4 children (the last having been weaned 10 months earlier) began lactating after 1 week of a cancer chemotherapy regimen for breast cancer that included tamoxifen 20 mg/day. Milk production continued for several weeks until tamoxifen was discontinued after which it did not return during 12 more weeks of chemotherapy.[3]

Tamoxifen Identification

Substance Name

Tamoxifen

CAS Registry Number

10540-29-1

Drug Class

Antineoplastic Agents

Estrogen Antagonists

Administrative Information

LactMed Record Number

250

Last Revision Date

20150310

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